doc/examples/vaapi_encode: Try to check fwrite() for failure

Fixes: CID1604548 Unused value

Sponsored-by: Sovereign Tech Fund
Reviewed-by: "Xiang, Haihao" <haihao.xiang-at-intel.com@ffmpeg.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 3e4bfff211)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/examples/vaapi_encode.c

@@ -88,6 +88,10 @@ static int encode_write(AVCodecContext *avctx, AVFrame *frame, FILE *fout)
         enc_pkt->stream_index = 0;
         ret = fwrite(enc_pkt->data, enc_pkt->size, 1, fout);
         av_packet_unref(enc_pkt);
+        if (ret != enc_pkt->size) {
+            ret = AVERROR(errno);
+            break;
+        }
     }
 
 end: